1. Field of the invention
The present invention relates to an apparatus and method for producing ultrapure water and a method of controlling the apparatus, and particularly to a high-efficiency apparatus and method for producing ultrapure water which are used for producing semiconductors and a method of controlling the apparatus.
2. Description of Related Art
FIG. 7 shows a principal portion of a conventional apparatus for producing ultrapure water in a primary pure water system (after pretreatment), which is most frequently used.
In the drawing, the primary pure water (referred to as "pretreated water" hereinafter) which is subjected to pretreatment is introduced, by piping 1, into an upper portion of a cation exchange resin tower 2 which is charged with a cation exchange resin 3. The lower end of the cation exchange resin tower 2 is connected to a decarbonation tower 5 by piping 4. The decarbonation tower 5 is connected to an anion exchange resin tower 8 charged with an anion exchange resin 9 by piping 10 through a pump 6. The anion exchange resin tower 8 is connected to a reverse osmosis membrane unit (referred to as "RO unit" hereinafter) provided with a reverse osmosis membrane by piping 14.
In the conventional apparatus for producing ultrapure water configured as described above, the pretreated water is first introduced into the cation exchange resin tower 2 by the piping 1. After cations such as Na.sup.+ ions and the like contained in the pretreated water introduced into the cation exchange resin tower 2 are adsorbed on the cation exchange resin 3 charged in the tower 2, the pretreated water is poured into the decarbonation tower 5 through the piping 4. In the decarbonation tower 5, CO.sub.2 is removed from the pretreated water. The pretreated water is then sent up to the upper portion of the anion exchange resin tower 8 from the decarbonation tower 5 by the pump 6 through the piping 7, 10. The anions in the pretreated water are replaced by the reactive groups of the anion exchange resin 9 charged in the anion exchange resin tower 8 and changed to OH.sup.- ions. The H.sup.+ supplied from the cation exchange resin and OH.sup.- supplied from the anion exchange resin form H.sub.2 O. In this way, most of charged impurities are removed.
The water treated in the anion exchange resin tower 8 is then poured, by the piping 14, into the RO unit 11 for removing charged and neutral impurities from the water. The thus-obtained ultrapure water is supplied to a use point from piping 13, and the water containing concentrated charged and neutral impurities is discharged from an discharge pipe 12.
The above-described apparatus for producing ultrapure water has the problem that charged and neutral impurities cannot be removed with high efficiency because they have a small particle size. Particularly, colloidal impurities cannot be easily removed because they have a size of 0.001 to 1 .mu.m and intermediate properties between the properties of ions and neutral particles. The apparatus also has the problem that if an attempt is made to remove such impurities having a small size, the load of treatment is applied to the after system, and the after system is excessively increased in its cost and space.